1. Field of the Invention
The present invention relates to a process for producing an aromatic imide polymer laminate material. More particularly, the present invention relates to a process for producing an aromatic imide polymer layer-metallic foil laminate material having excellent heat resistance, electric properties, and mechanical properties and, therefore, useful as a flexible circuit board substrate.
2. Description of the Prior Art
A "flexible circuit board substrate" is a flexible substrate for a printed circuit board. It is useful for producing simplified and/or highly dense printed circuit boards.
A typical, conventional flexible circuit board substrate is a composite material composed of copper foil and aromatic polyimide film bonded with the copper foil by a bonding material. This type of substrate is called a "polyimide-laminated copper substrate".
In a polyimide-laminated copper substrate, the polyimide film is bonded with the copper foil by a bonding material layer having a thickness of 10 to 30 microns. That is, the polyimide-laminated copper substrate is a three-layer laminate. Accordingly, the heat-resistance, electric properties, chemical resistance, and mechanical properties required of the polyimide-laminated copper substrate are determined mainly by the bonding material, not by the polyimide film. That is, the inherent excellent properties and advantages of the polyimide film are not fully exhibited in a conventional polyimide-laminated copper substrate due to the bonding material layer.
Under the above-mentioned circumstances, various attempts have been made to provide a new process for producing a flexible circuit board substrate without using bonding material. For example, U.S. Pat. No. 3,179,634 discloses a process wherein a solution of an aromatic polyamic acid, i.e., an aromatic polyamide acid which is a precursory compound of the corresponding aromatic polyimide, is directly coated on a surface of a copper foil and the polyamic acid solution layer is heated so as to imide-cyclize the polyamic acid into the corresponding polyimide, whereby a composite laminate composed of an aromatic polyimide coating layer bonded directly to a copper foil is obtained.
In the above-mentioned process, it was found that since the condensation conversion of the polyamide acid into the corresponding polyimide included a dehydration reaction, it results in reduction of the volume of the coating layer. This shrinkage of the coating layer causes significant bending (curling) of the resultant composite substrate. A curled substrate is useless as a flexible circuit board substrate. Therefore, the above-mentioned process is not satisfactory for the production of actual circuit board substrates.
In an approach for eliminating the above-mentioned disadvantages, Japanese Unexamined Patent Publication (Kokai) No. 49-129,862 discloses a process wherein a solution of a partially cyclized polyimideamide acid prepared by the reaction of a polyamide acid, which has been produced from pyromellitic dianhydride, with diphenylether-4,4'-diisocyanate is directly coated on a surface of a conductive metallic foil, such as copper foil, and the solution layer is heated so as to provide a completely cyclized polyimideamide layer. Compared with the process of the above-mentioned U.S. Patent this process is slightly more effective for decreasing the degree of curling of the resultant composite substrate. However, the resultant composite substrate is still curled and; therefore, is still unsatisfactory.